Sanji's flames are hotter than King's, because blue fire is hotter than Yellow white fire (as seen in the volume 102 cover) and Plasma is coming off of Ifrit Jambe. If Oda seriously wanted us to think King's was hotter he wouldn't have designed it that way. And the comparison made with Magma by Zoro is pretty shallow and the reason why Magma is above fire is for it being a solid and consuming fire. So yes Sanji's fire is hotter and there is no problem with the idea unless the idea of Sanji being strong makes you uncomfortable.